So something to keep in mind with the symptoms you posted is the parking sensor beeps pass through the audio system's ACM module which is almost always what fails in a 'frozen' state which causes these battery drain issues, no audio, no ability to change audio sources, etc.. Being frozen also means any voice alerts or beeps from Sync or in this case the parking sensor beeps don't get properly routed to the speakers in this state. So just a little bit of extra info to be aware of with this common issue.
So to try and explain what is going on in laymens terms is the audio system in our vehicles is a multi-module system. It's not like the old days where it was just the one single headunit doing everything. The following diagram is from Ford's own service manual. It seems to be missing a few small details.
The APIM is responsible for just stuff like Navigation, Bluetooth Audio, USB ports, and any alerts like voice notifications, touch 'tick' sound, parking sensor beeps, etc.. This is what you see. It is the touch screen interface and UI and all those tasks therein. The ACM just handles the AM/FM/Sirius tuners, and CD playback. Just like an old school head unit like the following. No user interface. It acts at the whim of the APIM.
Now the ACM has multiple audio INPUTS. One for the main media audio from the APIM so when you are listening to a Bluetooth audio device or playing from a USB flash drive, this will be used. Then there is a second audio input purely for alerts and voice notifications from Sync. This gets passed from an isolated source from the APIM to the ACM.
The ACM then acts as a glorified audio mixer and switcher at that point. When you change sources on the APIM, say from FM to Sirius or USB to FM, it is telling the APIM to switch inputs itself as needed and re-mix the audio to go out to the speakers at the correct desired volumes. When any alerts come up such as parking sensor beeps or turn by turn directions, the APIM signals this to the ACM over the Canbus while feeding said alerts on that isolated input mentioned above. The ACM then 'ducks' the main audio and mixes in the alerts at their set volume and out to the speakers.
So what happens in the no audio cases is quite simple. The ACM essentially locks up. Why this often ends up draining the battery is there is no longer an analog switched power line to tell it to turn off and off with the key. It is all communicated over the Canbus digitally. If the ACM is frozen it cannot shut off when told to do so even if the rest of the car is off. What also happens as you all experience is you may be on a given audio source that may still be audible. USB media can change tracks for example. Radio stations will likely be stuck but audible. But beyond that volume can't be changed, sources can't be changed, and audio can't be turned off or on. All that is handled by the ACM. And stuff like parking sensors, the touch screen 'tick' sound and other 'alerts' are being sent by the APIM but the ACM is not responding to any commands to duck the media audio and send those alerts out to the speakers so they are never heard in this state.
As has been covered ad nauseum in this forum, the easiest workaround which can be done in minutes is to pull the relevant fuses for the audio system for a few minutes and it'll resolve it for a while again. Basically giving it a hard reboot. If you ever see any of these odd symptoms with the audio system, anticipate needing to do this or risk a drained battery. It's as simple as that.
The system may seem a tad complex but it is also what has allowed Ford in the original inception of Sync to keep the costs insanely low compared to other existing infotainment systems on the market. No need to re-build existing headunits and the APIM is largely an early generation smartphone hardened for the automotive environment and the rest is software to have the various bits interact with each other in a cohesive manner.
Edited by cr08, 10 January 2019 - 05:54 PM.